#+BEGIN_LaTeX ... #+END_LaTeX only applies when exporting to latex. It allows for backend specific code when exporting (by the same concept #+Begin_HTML ... #+END_HTML does the same for HTML code.

The full set of environments that allow for LaTeX code to be generated on export can be found in the Org Manual.

LaTeX fragments don't need any special marking at all. The following
snippets will be identified as LaTeX source code:

Environments of any kind. The only requirement is that the \begin and \end statements appear on a new line, at the beginning of the line or after whitespaces only.

Text within the usual LaTeX math delimiters. To avoid conflicts with currency specifications, single ‘$’ characters are only recognized as math delimiters if the enclosed text contains at most two line breaks, is directly attached to the ‘$’ characters with no whitespace in between, and if the closing ‘$’ is followed by whitespace, punctuation or a dash. For the other delimiters, there is no such restriction, so when in doubt, use ‘(...)’ as inline math delimiters.